This invention relates to the field of preparing carbon fibers from carbonaceous pitches. A typical process for manufacturing pitch based carbon fibers may include the following steps: (1) preparing a suitable pitch for spinning; (2) spinning the pitch into as-spun pitch fibers; (3) thermosetting (stabilizing) the pitch fibers to render them infusible, i.e. unmeltable; and, (4) carbonizing the fibers by heating the stabilized fibers to carbonization temperatures.
In the described process, the as-spun pitch fiber of step (2) is a thermoplastic material. Thus, additional heating of the fiber results in melting and loss of fiber structure. Therefore, prior to carbonization, the fiber must be rendered unmeltable, i.e. thermoset. The thermosetting process is commonly known as oxidative stabilization due to the heating of the fiber in the presence of an oxidizing agent. Typical stabilization processes expose the as-spun fibers to a high concentration of oxidizing agent at an initial process temperature lower than the fiber's spinning temperature.
The stabilization process involves temperature dependent diffusion of oxygen into the fiber where the oxygen reacts with and promotes cross-linking of the pitch molecules. Because the reaction rate is temperature dependent, lower stabilization temperatures require longer times to complete the oxidative stabilization of the fiber. The total oxygen required for stabilization will depend on the nature of the pitch. Generally, low softening point pitches require long periods of time and more oxygen to complete the stabilization process. Typically, the oxidizing agent is air (approximately 21% oxygen).
To improve operating economics, one would prefer to stabilize (thermoset) the as-spun fiber at high temperatures under high oxygen concentrations in order to complete the stabilization process in the shortest period of time. Unfortunately, high oxygen concentrations and elevated temperatures increase the possibility of uncontrolled exothermic oxidation reactions. Reactions of this type are particularly hazardous when highly volatile hydrocarbons are present. Most current art practices minimize the risk of thermal runaway by limiting the processing temperature and quantity of exposed fiber.
In addition to the need to prevent an uncontrolled exothermic reaction and loss of carbon mass, the stabilization process must also preserve the structure of the fiber. Accordingly, the heating temperature must not exceed the fiber's softening point. Therefore, fibers prepared from soft, low melting pitches must be stabilized at lower temperatures than fibers prepared from hard, high melting pitches.
Clearly, when treating a large amount of fiber over a short period of time the current manufacturing methods have significant drawbacks. The need to limit temperature, oxidant concentration and quantity of fiber in the stabilization process creates higher than desirable costs, diminishes the value and strength of the fiber and creates obvious operating risks. In overcoming the deficiencies of the current processes, a preferred method would utilize a low concentration of oxidizing agent coupled with high temperature heating while avoiding the risk of thermal runaway and loss of fiber size. Preferably, such a method would yield stabilized fibers in a short period of time and generate increased operating efficiencies.
To achieve these goals, the present invention provides a process for stabilizing pitch fibers using low concentrations of oxidizing agent at high temperature in a short period of time. This novel process stabilizes the core of the fiber without excessive surface oxidation. Additionally, the current invention provides a pitch fiber which becomes stabilized at its core at a rate which is sufficient to preclude excess loss of carbon at the fiber's surface due to oxidation. Further, the fibers take up a minimal amount of oxygen. These and other benefits of the present invention are described in greater detail below. For the purposes of this disclosure, the terms "stabilizing" and "thermosetting" are used interchangeably.